Physical properties of water resistance, workability and the like at high temperature are required for a paint for metal, particularly paints for coating metal cans used for packing of refreshing beverage, processed foods, and the like. Organic solvent type paints such as epoxy/amino resins, acrylic/amino resins, polyester/amino resins, and the like which form excellent coating film by heat curing have conventionally been used as such paints for coating metals. However, in those solvent type paints, a large amount of organic solvents volitilizes in atmosphere in coating them, causing the problems of environmental pollution, consumption of resourses, and the like.
In view of the above, it is considered to use an aqueous paint using water as a medium in place of the solvent type paints. However, the conventional aqueous paints have poor water resistance. Further, although the conventional aqueous paints are durable to hot water treatment of, for example, boiling water, if those are subjected to a treatment with a pressurized steam of 120.degree. C. or more (retort treatment), physical properties of the coating film thereof markedly deteriorate. Thus, the conventional aqueous paints cannot practically be used.
There are various proposals regarding aqueous paints which are durable to the pressurized steam treatment. For example, JP-A-3-72577 (The term "JP-A" used herein means an "unexamined published Japanese patent application) proposes an aqueous paint comprising (a) an aqueous acrylic resin obtained by neutralizing a copolymer having carboxyl group of the molecule, comprising an aromatic vinyl monomer and an alkyl (meth)acrylate as the main components, with a base, (b) a hydroxyl-containing polyol, and (c) an amino resin. However, the physical properties of the coating film obtained do not still reach to a practically satisfied level.
JP-A-6-65535 proposes an aqueous paint which comprises a macromonomer which is a polymer of (meth)acrylic acid alkyl ester monomers and/or styrene monomer units, as the main components, having a radically polymerizable group at one end thereof, and other copolymerizable monomer. However, the physical properties of the coating film thereof after a severe treatment of a retort treatment are not yet satisfactory.